In a mobile communication system, a user equipment may receive information from a base station through a downlink, and may also transmit information through an uplink. Examples of information transmitted from and received by the user equipment include data and various kinds of control information. Various physical channels exist depending on types of information transmitted from or received by the user equipment.
FIG. 1 is a diagram illustrating physical channels used in a 3rd generation partnership project long term evolution (3GPP) system, which is an example of a mobile communication system, and a general method for transmitting a signal using the physical channels.
A user equipment performs initial cell search such as synchronizing with a base station when it newly enters a cell or the power is turned on, at step S101. To this end, the user equipment may synchronize with the base station by receiving a primary synchronization channel (P-SCH) and a secondary synchronization channel (S-SCH) from the base station, and may acquire information of cell ID, etc. Afterwards, the user equipment may acquire broadcast information within a cell by receiving a physical broadcast channel from the base station. Meanwhile, the user equipment may identify the channel status of a downlink by receiving a downlink reference signal (DL RS) in the initial cell search step.
The user equipment which has finished the initial cell search may acquire more detailed system information by receiving a physical downlink control channel (PDCCH) and a physical downlink control channel (PDSCH) based on the physical downlink control channel information, at step S102.
Meanwhile, the user equipment that has not accessed the base station may perform a random access procedure (RACH) for the base station, such as step S103 to S106, to completely access the base station. To this end, the user equipment may transmit a preamble of a specific sequence through a random physical random access channel (PRACH) (S103), and may receive a response message to the random access through the PDCCH and a PDSCH corresponding to the PDCCH (S104). In case of contention based random access except for handover, a contention resolution procedure such as transmission (S105) of additional PRACH and reception (S106) of the PDCCH and the PDSCH corresponding to the PDCCH may be performed.
The user equipment which has performed the aforementioned steps may receive the PDCCH/PDSCH (S107) and transmit a physical uplink shared channel (PUSCH) and a physical uplink control channel (PUCCH) (S108), as a general procedure of transmitting uplink/downlink signals.
FIG. 2 is a diagram illustrating a signal processing procedure for transmitting an uplink signal from a user equipment.
A scrambling module 210 of a user equipment may scramble transmitting signals by using a user equipment specific scrambling signal to transmit an uplink signal. The scrambled signals are input to a modulation mapper 220 and are modulated into complex symbols by a binary phase shift keying (BPSK) mode, a quadrature phase shift keying (QPSK) mode, or a 16 quadrature amplitude modulation (QAM) mode depending on types of the transmitting signals and/or the channel status. Afterwards, the modulated complex symbols are processed by a conversion precoder 230 and then input to a resource element mapper 240. The resource element mapper 240 may map the complex symbols into time-frequency resource elements to be used for actual transmission. In this way, the processed signals may be transmitted to the base station through an antenna after passing through an SC-FDMA signal generator 250.
FIG. 3 is a diagram illustrating a signal processing procedure for transmitting a downlink signal from a base station.
In the 3GPP LTE system, the base station may transmit one or more codewords to the downlink. Accordingly, one or more codewords may be processed as complex symbols through a scrambling module 301 and a modulation mapper 302 in the same manner as the uplink of FIG. 2. Afterwards, the complex symbols are mapped into a plurality of layers by a layer mapper 303, wherein each layer may be multiplied by a predetermined precoding matrix selected by a precoding module 304 depending on the channel status and then may be allocated to each transmitting antenna. The transmitting signals per antenna, which are processed as above, are mapped into time-frequency resource elements to be used for transmission by a resource element mapper 305. Afterwards, the processed signals may be transmitted through each antenna after passing through an OFDM signal generator 306.
If the user equipment transmits a signal to the uplink in the mobile communication system, a peak-to-average-ratio (PAPR) ratio may cause a problem as compared with that the base station transmits a signal to the downlink. Accordingly, as described with reference to FIG. 2 and FIG. 3, SC-FDMA (Single Carrier-Frequency Division Multiple Access) system is used for uplink signal transmission unlike OFDMA system used for downlink signal transmission.
FIG. 4 is a diagram illustrating an SC-FDMA system for uplink signal transmission and an OFDMA system for downlink signal transmission in a mobile communication system.
Each of a user equipment for uplink signal transmission and a base station for downlink signal transmission includes a serial-to-parallel converter 401, a subcarrier mapper 403, an M-point IDFT module 404, and a cyclic prefix (CP) addition module 406.
However, the user equipment for signal transmission based on the SC-FDMA system further includes a parallel-to-serial converter 405 and an N-point DFT module 402. The N-point DFT module 402 offsets IDFT processing effect of the M-point IDFT module 404 as much as a predetermined portion, whereby the transmitting signals have single carrier properties.
FIG. 5 is a diagram illustrating a signal mapping system on a frequency domain for satisfying single carrier properties in the frequency domain. In FIG. 5, (a) illustrates a localized mapping system, and (b) illustrates a distributed mapping system. In the current 3GPP LTE system, the localized mapping system is defined.
Meanwhile, a clustered SC-FDMA which is a corrected type of SC-FDMA will be described. The clustered SC-FDMA divides DFT process output samples into sub-groups in sequentially mapping subcarriers between DFT process and IFFT process, and maps the DFT process output samples into subcarrier regions, which are spaced apart from one another per sub-group in an IFFT sample input module. The clustered SC-FDMA may include a filtering process and a cyclic extension process as the case may be.
At this time, the sub-group may be designated as a cluster. The cyclic extension process means that a guard interval longer than maximum delay spread of a channel is inserted between contiguous symbols to avoid inter-symbol interference while each symbol of subcarriers is being transmitted through a multi-path channel.